Field of the Invention
The present invention concerns a method of forming hidden color patterns, such as text or images, on porous substrates. Particularly, the invention concerns a method for manufacturing patterned porous substrates by forming hydrophobic patterns on a hydrophilic surface, the formed patterned porous substrates, and a method for bringing said pattern into a visible state.
Description of Related Art
In many porous substrates, such as nitrocellulose sheets, cellulose-based papers, and porous polymer sheets, liquids travel laterally along the substrate sheet. The flow is generally capillary. Such sheets and their liquid flow are exploited in many applications in the field of diagnostics, such as in biosensors and immunoassay-lateral-flows. In these applications, a strip has been used, in which the liquid travels laterally along the entire width of the strip, cut from a substrate sheet. In multi-analysis-tests, in which the sample liquid must be transported to several reaction/detection areas, it is advantageous for it to be possible to form the substrate sheet in such a way that the sample liquid travels in only specific parts of the sheet, i.e. structural layers guiding the liquid flow are formed in the sheet.
Such structural layers guiding the liquid flow can be manufactured in porous substrate sheets using many different methods (see e.g. US 2009/0298191 A1), such as the following methods, wherein:                A substrate sheet is saturated with a photoresist, exposed to UV light through a photo-mask defining the liquid channels, and finally developed, when the photoresist is dissolved off the locations of the liquid channels. In this way, areas saturated with photoresist are created, which define the edges of the liquid channels.        A hardening polymer, e.g., polydimethylsiloxane (PDMS), is spread on a stamp, the relief pattern of which defines the boundary areas of the liquid channels. After this, the stamp is pressed onto the substrate sheet, for example, for 20 seconds. Finally, the stamp is removed and the polymer is hardened.        Liquids, which are either hydrophobic themselves, or which can convert the substrate sheet to become hydrophobic, can be applied on the substrate sheet according to a desired pattern, for example, using the following methods: spraying the liquid through a stencil, by silkscreen printing, by inkjet printing, or by using a plotter.        The desired areas of the substrate are saturated to become hydrophobic by absorbing wax with the aid of heat.        
In the publication by D. A. Bruzewicz, M. Reches, and G. M. Whitesides (‘Low-cost printing of poly(dimethylsiloxane) barriers to define microchannels in paper’, Anal. Chem., 2008, 80 (9), 3387-3392), barrier lines guiding the liquid flow are manufactured using a PDMS solution as an ink in the pen of a plotter.
With the exception of the photoresist-based method, the precision of the edges of the liquid-flow channels are a problem in the aforementioned methods according to the prior art. Because the liquid, which alters the substrate sheet in such a way as to guide a liquid flow, must be absorbed through the entire substrate sheet, it also spreads at the same time laterally and thus the edges of the liquid-flow channel do not become precise.
The publication K. Abe, K. Suzuki, and D. Citterio, ‘Inkjet-printed microfluidic multianalyte chemical sensing paper’, Anal. Chem., 80 (18), 6928-6934, 2008, discloses a method, in which the paper in first saturated with a 1.0 w-% polystyrene-toluene solution, dried, and the liquid channels are finally etched open by inkjet printing with toluene. The inkjet printing generally has to be repeated 10-30 times to achieve a sufficient etching depth, which makes it difficult to use the method in roller-to-roller manufacturing processes.
All of the aforementioned manufacturing methods according to the prior art are quite slow and thus difficult to use in industrial mass-manufacturing processes. In US 2009/0298191 A1, it is estimated that patterning a single 10×10 cm substrate sheet using a photoresist-based method takes about 8-10 minutes and with a method using a stamp about 2 minutes.
Crayola produces a product “Color Wonder”, which is a paper coating, which reacts with “invisible” ink in such a way that color is formed. This color change has the disadvantage of being permanent. Further, the system is based on a specially developed paper coating, and is expensive to produce.
Bruynzeel-sakura produces a product “COLOUR WITH WATER” (e.g. http://webshop.bruynzeel-sakura.com), which consists of a white paper coating on a defined area, which becomes transparent upon addition of liquids such as water. The shape of the image in the system is visible prior to addition of water, as the form of the coating defines the area that becomes transparent.